But in case of Keplers I really don't know where to dig - nvidia drivers, card configs, vga bios difference or something else.
Hay, What do you think about looking at how the displays are configured? I ask because I've been trying to get a K3000m in a Dell M6700 working. I've experienced much the same as you. However I've found that the internal LCD works, and with CE/CI, but the backlight is off. I can connect using desktop sharing and see in ioreg that the card is initialized and working. I can also change resolutions. So it's getting the internal LCD's EDID okay.
However when I connect an external display on any port it hangs the graphics system(I can still SSH in).
I also read that Kepler cards for the first time support 4 displays simultaneously. This implies some changes in the way the displays are connected and defined. Could it be that different things need to be injected for this new setup? One anomaly I noticed on the M6700, all the 4 displays have a 'port-number' property set to 9. Looking at the ioreg from an nvidia retina mac. It's port-numbers are 0, 1, 2, 3. If this is the 'port' in the same meaning as with Intel HD graphics. There is no way they should be the same.